Semiconductor optical device and an optical processing system that uses such a semiconductor optical system

ABSTRACT

A photodetection device includes a collector layer, a collector electrode connected electrically to the collector layer, a base layer free from a junction region for contacting with an electrode, an emitter layer including at least two, mutually separated emitter regions; and at least two emitter electrodes provided respectively on the emitter regions, wherein the base layer is exposed optically to an external optical radiation.

BACKGROUND OF THE INVENTION

The present invention generally relates to the art of opticalinformation processing and more particularly to an optical semiconductordevice as well as to an optical processing system that uses such anoptical semiconductor device.

With the persisting demand for high speed semiconductor devices,intensive efforts have been made for developing high speed compoundsemiconductor devices and integrated circuits thereof that have anoperational speed exceeding the operational speed of conventional Sidevices. A hot electron transistor (HET) is a representative example ofsuch a high speed compound semiconductor device. Particularly, it isnoted that there is a proposal for a multiple emitter HET that includesa plurality of emitters.

Further, the inventor of the present invention has proposed a multipleemitter heterobipolar transistor (HBT) having a structure somewhatsimilar to that of the multiple emitter HET in the U.S. Pat. No.5,561,306 patent application Ser. No. 08/295,538). In the proposeddevice, the base electrode is eliminated, and two different emitterregions are defined on a common emitter layer for carrying first andsecond emitter electrodes respectively. Thereby, one of the emitterelectrodes is used for removing holes accumulated in the base layer. Inthe proposed device, the impurity concentration level of the n-typeemitter is set to about 2×10¹⁸ cm⁻³, which is substantially larger thanthe effective density of state of electrons (≈5×10¹⁷ cm⁻³) in theemitter. Simultaneously, the impurity concentration level of the p-typebase is set to about 1×10¹⁹ cm⁻³, which is substantially larger than theeffective density of state of holes (≈1×10¹⁹ cm⁻³) in the base. Bysetting the impurity concentration level of the emitter and base assuch, it is possible to reduce the base resistance of the devicesignificantly by removing the holes from the base by tunneling throughthe depletion region formed at the base-emitter junction, and oneobtains a high speed semiconductor device. It should be noted that sucha HBT has a distinct advantage over conventional HETs (that operate onlyin a very low temperature environment) they are operational also at aroom temperature environment.

Meanwhile, there is a rapid development in the field of super high speedoptical telecommunication networks, particularly in relation to theso-called multimedia applications. In order to realize a high speed,large capacity telecommunication network, it is not only necessary touse optical fibers in place of electric cables but also necessary toincrease the transmission rate of the optical fibers from theconventional rate of 1 Gbit/s to at least 4 Gbit/s, and preferablylarger than 10 Gbit/s.

When the transmission rate of the optical signals through the opticalfiber network is increased as such, there naturally arises a demand fora photoreception device that responds to such a very high speed opticalsignals. Further, there exists various demands for other associatedcircuits such as a high speed preamplifier for amplifying the highfrequency electric signals obtained by the photoreception device or ademultiplexer for extracting signals of desired channels from amultiplexed optical signal by conducting a demultiplexing process andfor transmitting the same with a lower transmission rate.

FIG. 1 shows the construction of a conventional optical receiverincluding a demultiplexer.

Referring to FIG. 1, the optical receiver includes a photoreceptiondevice 103 such as a PIN diode that receives an optical signal from anoptical transmitter 101 via an optical fiber 102, wherein thephotoreception device 103 produces an electric output signal andsupplies the same to a preamplifier 104 typically formed of a high speedtransistor for amplification. The electric signal thus amplified in turnis supplied to a demultiplexer 105 for demultiplexing into electricsignals of respective channels.

Generally, such an optical receiver has a drawback in that it requires acomplex construction. Further, there is a drawback in that, while thePIN photodiode used for the photoreception device may have asufficiently high response speed, the overall response of the opticalreceiver may be limited by the response of the preamplifier 104 or thedemultiplexer 105. Thus, there has been a demand for a high speedoptical receiver that has a simple construction and provides a highresponse speed suitable for processing high speed optical signals.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean optical semiconductor device as well as an integrated circuitthereof, wherein the foregoing problems are successfully eliminated.

Another and more specific object of the present invention is to providea high speed optical semiconductor device having a simple structurebased upon a multiple emitter HBT as well as an optical integratedcircuit thereof.

Another object of the present invention is to provide a signalextraction or sampling process that uses a high speed opticalsemiconductor device based upon a multiple emitter HBT.

Another object of the present invention is to provide an electro-opticallogic gate that performs a logic operation of an input optical signaland an input electric signal both having a logic level.

Another object of the present invention is to provide a light emittingdevice, comprising:

a collector layer;

a collector electrode connected electrically to said collector layer;

a base layer provided on said collector layer, said base layer beingfree from a junction region for contacting with an electrode;

an emitter layer provided on said base layer, said emitter layerincluding at least two, mutually separated emitter regions; and

at least two emitter electrodes provided respectively on said at leasttwo emitter regions.

According to the present invention, it is possible to cause an opticalemission at the p-n junction formed between each of the emitter regionsand the base layer by applying a voltage across the plurality of emitterregions, such that a recombination of electrons and holes occurs at thep-n junction. As the thin base layer is free from a base electrode, thefabrication of the device is substantially facilitated.

Another object of the present invention is to provide a photodetectiondevice, comprising:

a collector layer;

a collector electrode connected electrically to said collector layer;

a base layer provided on said collector layer, said base layer beingfree from a junction region for contacting with an electrode;

an emitter layer provided on said base layer, said emitter layerincluding at least two, mutually separated emitter regions; and

at least two emitter electrodes provided respectively on said at leasttwo emitter regions;

said base layer being exposed optically to an external opticalradiation.

According to the present invention, it is possible to detect an opticalbeam incident to the base layer by detecting a current flowing betweenthe plurality of emitter regions. In response to the optical radiationapplied to the base layer of the device, electrons and holes are excitedin the base layer, wherein the thus excited electrons escape immediatelyto the emitter regions, leaving an accumulation of holes in the baselayer. With an increasing degree of accumulation of holes in the baselayer, the energy level of the base layer shifts in the lower directionwith respect to the emitter regions for both the conduction band andvalence band. Ultimately, the energy level of the valence band in thebase layer becomes equal to the energy level of the valence band of theemitter regions, and the holes cause an escape from the base layer tothe emitter regions, and a current flows between the plurality ofemitter regions.

Alternatively, one may apply a bias voltage between one of the emitterregions and the collector layer for detecting the incident optical beam.In the latter operational mode, the electrons excited in the base layeras a result of optical radiation escape immediately to the collectorlayer to form a collector current, leaving an accumulation of holes inthe base layer. In the present invention, one can annihilate suchaccumulation of holes in the base layer by injecting electrons into thebase layer from another emitter region. Thereby, the photodetectiondevice recovers the original operational state immediately whenever theincident optical beam is interrupted.

In any of the foregoing operational modes, the thin base layer is freefrom a base electrode. Thus, the fabrication of the device issubstantially facilitated.

Another object of the present invention is to provide a method fordetecting an optical beam by a photodetection device, saidphotodetection device including: a collector layer; a collectorelectrode connected electrically to said collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions; and at least two emitter electrodesprovided respectively on said at least two emitter regions; said baselayer being exposed optically to an external optical radiation, saidmethod comprising the steps of:

applying a bias voltage across two of said emitter electrodes; and

applying an optical beam to said base layer as said external opticalradiation, with an optical energy exceeding a bandgap of said baselayer.

Another object of the present invention is to provide a method fordetecting an optical beam by a photodetection device, saidphotodetection device including: a collector layer; a collectorelectrode connected electrically to said collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions; and at least two emitter electrodesprovided respectively on said at least two emitter regions; said baselayer being exposed optically to an external optical radiation, saidmethod comprising the steps of:

applying a bias voltage across one of said emitter electrodes and saidcollector electrode;

applying an optical beam to said base layer as said external opticalradiation, with an optical energy exceeding a bandgap of said baselayer; and

after interrupting said optical beam, injecting electrons from anotheremitter electrode to said base layer so as to cancel out accumulation ofholes in said base layer.

According to the present invention, one can annihilate such accumulationof holes in the base layer by injecting electrons into the base layerfrom another emitter region. Thereby, the photodetection device recoversthe original operational state immediately whenever the incident opticalbeam is interrupted.

In any of the foregoing operational modes, the thin base layer is freefrom a base electrode. Thus, the fabrication of the device issubstantially facilitated.

Another object of the present invention is to provide an electro-opticallogic device, comprising:

a semiconductor device having a collector layer, a collector electrodeconnected electrically to said collector layer, a base layer provided onsaid collector layer, said base layer being free from a junction regionfor contacting with an electrode, an emitter layer provided on said baselayer, said emitter layer including at least two, mutually separatedemitter regions, and at least two emitter electrodes providedrespectively on said at least two emitter regions, said base layer beingexposed optically to an external optical radiation;

an optical window provided on said semiconductor device for injecting aninput optical logic signal to said base layer; and

an input terminal connected to one of said emitter electrodes forsupplying an input electric logic signal thereto; and

an output terminal connected to said collector layer.

According to the present embodiment, one can achieve a logic operationsuch as a logic product or a logic sum between electric and opticallogic signals.

Another object of the present invention is to provide an electro-opticalsampling device, comprising:

a semiconductor device having a collector layer, a collector electrodeconnected electrically to said collector layer, a base layer provided onsaid collector layer, said base layer being free from a junction regionfor contacting with an electrode, an emitter layer provided on said baselayer, said emitter layer including at least two, mutually separatedemitter regions, and at least two emitter electrodes providedrespectively on said at least two emitter regions, said base layer beingexposed optically to an external optical radiation;

an optical window provided on said semiconductor device for injecting aninput optical signal to said base layer; and

an input terminal connected to one of said emitter electrodes forsupplying an electric clock signal thereto; and

an output terminal connected to said collector layer.

According to the present invention, it is possible to carry out samplingof an input optical signal in response to an optical clock signal athigh speed while using a simple construction for the sampling device.

Another object of the present invention is to provide an electro-opticalsampling device, comprising:

a semiconductor device having a collector layer, a collector electrodeconnected electrically to said collector layer, a base layer provided onsaid collector layer, said base layer being free from a junction regionfor contacting with an electrode, an emitter layer provided on said baselayer, said emitter layer including at least two, mutually separatedemitter regions, and at least two emitter electrodes providedrespectively on said at least two emitter regions, said base layer beingexposed optically to an external optical radiation;

an optical window provided on said semiconductor device for injecting anoptical clock signal to said base layer;

an input terminal connected to one of said emitter electrodes forsupplying an input electric signal thereto; and

an output terminal connected to said collector layer.

According to the present invention, it is possible to sample an electricinput signal in response to an optical clock signal at high speed whileusing a very simple construction for the sampling device.

Another object of the present invention is to provide an opticaldemultiplexer, comprising:

a plurality of optical semiconductor devices each including a collectorlayer, a base layer provided on said collector layer and an emitterlayer provided on said base layer, said emitter layer including a firstemitter region and a second, different emitter region;

input optical waveguide means for supplying an input electric signal tosaid base layer of each of said optical semiconductor devices;

each of said first emitter regions of said plurality of opticalsemiconductor devices being grounded commonly; and

a plurality of output terminals connected to respective collector layersof said optical semiconductor devices;

each of said second emitter regions of said plurality of opticalsemiconductor devices being supplied with a corresponding electric pulsesignal for producing an output signal at said collector thereof.

According to the present invention, it is possible to carry out atime-divisional demultiplexing of an input optical signal having a hightransmission speed by a demultiplexer that has a very simpleconstruction.

Another object of the present invention is to provide an opticallycontrolled multiplexer, comprising:

a plurality of optical semiconductor devices each comprising a collectorlayer, a base layer provided on said collector layer and an emitterlayer provided on said base layer, said emitter layer including a firstregion and a second, different region;

a plurality of input optical waveguides provided in correspondence toplurality of channels, each of said input optical waveguides supplyingan optical signal of the base layer of a corresponding one of saidoptical semiconductor devices;

in each of said plurality of optical semiconductor devices, said firstregion being connected to the ground;

in each of said plurality of optical semiconductor devices, said secondregion being supplied with a corresponding electric timing pulse; and

an output terminal connected commonly to said collector layers of saidoptical semiconductor devices.

According to the present invention, it is possible to produce amultiplex optical signal by a demultiplexer having a very simpleconstruction.

Another object of the present invention is to provide an electro-opticalsampling device, comprising:

a first bipolar transistor comprising a collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; and an emitter layerprovided on said base layer, said emitter layer including at least firstand second, mutually different emitter regions, said base layer of saidoptical semiconductor device being adapted for receiving an inputoptical signal in the form of an optical beam;

a second bipolar transistor comprising a collector layer, a base layerprovided on said collector layer, and an emitter layer provided on saidbase layer, said emitter layer including at least first and second,mutually different emitter regions, said base layer being free from ajunction region for contacting with an electrode;

a power feed path connected commonly to said first emitter region ofsaid first bipolar transistor and said emitter layer of said secondbipolar transistor for supplying electric power to both said firstemitter regions via a common current source;

an input terminal connected to said second emitter region of said firstbipolar transistor for supplying thereto an input electric signal;

a ground path connected commonly .to said collector of said fist bipolartransistor and said collector of said second bipolar transistor forgrounding the same;

an input optical path for supplying an input optical signal to the baselayer of each of said first and second bipolar transistors;

an output terminal connected to said collector layer of one of saidfirst and second bipolar transistors.

According to the present invention, it is possible to carry out asampling of an optical signal in response to an electric clock signalwhile minimizing the offset in the output that appears when only one ofthe optical signal and the electric signal is supplied.

Another object of the present invention is to provide an electro-opticalsampling device, comprising:

a first bipolar transistor comprising a collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; and an emitter layerprovided on said base layer, said emitter layer including at least firstand second, mutually different emitter regions, said base layer of saidoptical semiconductor device being adapted for receiving an inputoptical signal in the form of an optical beam;

a second bipolar transistor comprising a collector layer, a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode, and an emitter layerprovided on said base layer;

a power feed path connected commonly to said first emitter region ofsaid first bipolar transistor and said emitter layer of said secondbipolar transistor for supplying electric power to both said firstemitter regions via a common current source;

an input terminal connected to said second emitter region of said firstbipolar transistor for supplying thereto an input electric signal;

a ground path connected commonly to said collector of said first bipolartransistor and said collector of said second bipolar transistor forgrounding the same;

an input optical path for supplying an input optical signal to the baselayer of each of said first and second bipolar transistors;

an output terminal connected to said collector layer of one of saidfirst and second bipolar transistors.

According to the present invention, it is possible to carry out asampling of an optical signal in response to an electric clock signalwhile minimizing the offset in the output that appears when only one ofthe optical signal and the electric signal is supplied, similarly asbefore.

Another object of the present invention is to provide a signal samplingmethod, comprising the steps of:

supplying an optical signal to an optical semiconductor device, saidoptical semiconductor device responding to both of an input electricsignal and an input optical signal supplied thereto;

supplying an electric signal to said optical semiconductor device;

said optical semiconductor device being the one that produces a firstoutput electric signal when both of said electric signal and opticalsignal are supplied, said optical semiconductor device further producinga second, different output electric signal when one or both saidelectric signal and said optical signal are not supplied;

said optical semiconductor device thereby sampling one of said opticalsignal and said electric signal in response to the other of said opticalsignal and said electric signal.

According to the present invention, one can achieve the desired samplingof a high speed input optical signal in response to an optical signal.

Another object of the present invention is to provide a method forsampling an input electric signal in response to an optical samplingpulse, comprising the steps of:

supplying electric power to a collector of a bipolar transistor, saidbipolar transistor further having an open base and at least first andsecond emitters;

grounding said first emitter;

supplying said input electric signal to said second emitter with afrequency nf₀, with the parameter n being set to be an integer;

supplying said optical sampling pulse to said base layer with a clockfrequency of f₀ -.sub.Δ f where .sub.Δ f is set to satisfy thenonequality .sub.Δ f<<f₀.

According to the present invention, one can achieve a sampling of aninput optical signal by a low frequency electric sampling pulse with afrequency asynchronous to the signal frequency of the optical signal.Thereby, one can extract or demodulate low frequency signal componentsfrom the input optical signal.

Another object of the present invention is to provide a method forsampling an input optical signal in response to an electric samplingpulse, comprising the steps of:

supplying electric power to a collector of a bipolar transistor, saidbipolar transistor further having an open base and at least first andsecond emitters;

grounding said first emitter;

supplying said electric sampling pulse to said second emitter with afrequency nf₀, with the parameter n being set to be an integer;

supplying said optical input signal to said base layer with a clockfrequency of f₀ -.sub.Δ f where .sub.Δ f is set to satisfy thenonequality .sub.Δ f<<f₀.

According to the present invention, one can achieve a sampling of aninput optical signal by a low frequency electric sampling pulse with afrequency asynchronous to the signal frequency of the optical signal.Thereby, one can extract or demodulate low frequency signal componentsfrom the input optical signal.

Another object of the present invention is to provide a method forobtaining a logic product of an input optical signal and an inputelectric signal, comprising the steps of:

supplying said input optical signal to a base of an optical bipolartransistor provided on a collector with an optical power selected fromone of first and second logic levels, said optical bipolar transistorfurther including at least first and second emitters;

applying said input electric signal across said first and secondemitters with a magnitude selected from one of first and second logiclevels; and

detecting a base current flowing between said first and second emitters;

said step of supplying said input optical signal and said step ofapplying said input electric signal being conducted by setting saidfirst and second logic levels of said input optical signal and saidoutput optical signal such that said base current flows only when bothof said input optical signal and said output optical signal have saidsecond logic level.

According to the present invention, it is possible to construct anelectro-optical AND gate from a single multiple emitter HBT.

Another object of the present invention is to provide a method forobtaining a logic product of an input optical signal and an inputelectric signal, comprising the steps of:

supplying said input optical signal to a base of an optical bipolartransistor provided on a collector with an optical power selected fromone of first and second logic levels, said optical bipolar transistorfurther including at least first and second emitters;

applying said input electric signal to one of said first and secondemitters with a magnitude selected from one of first and second logiclevels; and

detecting a collector current;

said step of supplying said input optical signal and said step ofapplying said input electric signal being conducted by setting saidfirst and second logic levels of said input optical signal and saidoutput optical signal such that said collector current flows only whenboth of said input optical signal and said output optical signal havesaid second logic level.

According to the present invention, it is possible to construct anelectro-optical AND gate from a single multiple emitter HBT.

Another object of the present invention is to provide a method forobtaining a logic sum of an input optical signal and an input electricsignal, comprising the steps of:

supplying said input optical signal to a base of an optical bipolartransistor provided on a collector with an optical power selected fromone of first and second logic levels, said optical bipolar transistorfurther including at least first and second emitters;

applying said input electric signal to one of said first and secondemitters with a magnitude selected from one of first and second logiclevels; and

detecting a collector current;

said step of supplying said input optical signal and said step ofapplying said input electric signal being conducted by setting saidfirst and second logic levels of said input optical signal and saidoutput optical signal such that said collector current flows only aslong as one of said input optical signal and said output optical signalhas said second logic level.

According to the present invention, it is possible to construct anelectro-optical OR gate from a single multiple emitter HBT.

Another object of the present invention is to provide a semiconductordevice, comprising:

a plurality of bipolar transistors arranged in rows and columns, each ofsaid bipolar transistors comprising:

a collector layer; a base layer provided on said collector layer, saidcollector layer being free from a junction region for contacting with anelectrode; and an emitter layer provided on said base layer, saidemitter layer including a first emitter region and a second emitterregion; and

optical waveguide means for supplying optical signals such that, in eachrow of said bipolar transistors, an optical signal is supplied commonlyto said base layers; and

said first emitter regions of said plurality of bipolar transistorsbeing connected commonly to the ground;

in each column of said plurality of bipolar transistors, said secondemitter regions being connected commonly to an input terminal;

in each row of said plurality of bipolar transistors, said collectorlayers being connected commonly to a power supply terminal.

According to the present invention, it is possible to construct anoptical read-only memory device by selectively providing an optical maskto one or more HBT elements in the array.

Another object of the present invention is to provide a semiconductoroptical integrated circuit, comprising:

a light emitting device including a collector layer, a collectorelectrode connected electrically to said collector layer, a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode, an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions, and at least two emitter electrodesprovided respectively on said at least two emitter regions, said lightemitting device producing an optical beam in said base layer thereof;and

a photodetection device, comprising: a collector layer, a collectorelectrode connected electrically to said collector layer, a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode, an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions, and at least two emitter electrodesprovided respectively on said at least two emitter regions, saidphotodetection device detecting an optical beam incident to said baselayer thereof;

said light emitting device and said photodetection device being providedon a common substrate.

Another object of the present invention is to provide a semiconductoroptical switch system, comprising:

a semiconductor light emitting device for emitting an optical beam; and

a semiconductor photodetection device disposed for detecting saidoptical beam;

said semiconductor optical switch circuit comprising a collector layer,a base layer provided on said collector layer, said base layer beingfree from a junction region for contacting with an electrode, and anemitter layer provided on said base layer, said emitter layer includingat least two, mutually separated emitter regions;

said semiconductor photodetection device comprising a collector layer, abase layer provided on said collector layer, said base layer being freefrom a junction region for contacting with an electrode, and an emitterlayer provided on said base layer, said emitter layer including at leasttwo, mutually separated emitter regions.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the construction of a conventionaltransmission-reception system that uses a time-divisional multiplexing;

FIG. 2 is a diagram for explaining the principle of a light emittingdevice according to the present invention;

FIGS. 3A and 3B are diagrams for explaining the principle of aphotodetection device according to the present invention;

FIG. 4 is a diagram showing the construction of a light emitting deviceaccording to a first embodiment of the present invention in a crosssectional view;

FIG. 5 is a diagram showing the construction of a light emitting deviceaccording to a second embodiment of the present invention in a crosssectional view;

FIGS. 6A-6C are diagrams showing the fabrication process of the deviceof FIG. 5;

FIG. 7 is a diagram showing the construction of a photodetection deviceaccording to a third embodiment of the present invention in a crosssectional view;

FIG. 8 is a diagram showing the construction of a photodetection deviceaccording to a fourth embodiment of the present invention in a crosssectional view;

FIG. 9 is a diagram showing the construction of a photodetection deviceaccording to a fifth embodiment of the present invention in a crosssectional view;

FIG. 10 is a diagram showing the construction of a photodetection deviceaccording to a sixth embodiment of the present invention in a crosssectional view;

FIG. 11 is a diagram showing the construction of a photodetection deviceaccording to a seventh embodiment of the present invention in a crosssectional view;

FIG. 12 is a diagram showing the construction of a photodetection deviceaccording to an eighth embodiment of the present invention in a planview;

FIG. 13 is a diagram showing the construction of a photodetection deviceaccording to a ninth embodiment of the present invention in a crosssectional view;

FIGS. 14A-14E are band diagrams showing the operational principle of thephotodetection device according to a tenth embodiment of the presentinvention;

FIGS. 15A-15C are band diagrams showing the operation of a conventionalphotodiode;

FIGS. 16A-16C are band diagrams showing the operation of a photodiodeaccording to an eleventh embodiment of the present invention;

FIG. 17 is a diagram showing the construction twelfth embodiment of thepresent invention in a cross of a semiconductor optical switch accordingto a sectional view;

FIG. 18 is a diagram showing the construction of a semiconductor opticalswitch according to a thirteenth embodiment of the present invention ina cross sectional view;

FIG. 19 is a diagram showing the construction of an opticalsemiconductor integrated circuit according to a fourteenth embodiment ofthe present invention in a cross sectional view;

FIG. 20 is a diagram showing the construction of an opticalsemiconductor integrated circuit according to a fifteenth embodiment ofthe present invention;

FIG. 21 is a diagram showing the construction of an opticalsemiconductor integrated circuit according to a sixteenth embodiment ofthe present invention;

FIG. 22 is a diagram showing the construction of an opticalsemiconductor integrated circuit according to a seventeenth embodimentof the present invention;

FIG. 23 is a diagram showing the overall construction of an opticaldemultiplexer according to an eighteenth embodiment of the presentinvention;

FIGS. 24A and 24B are diagrams respectively showing an equivalentcircuit diagram and an operational waveform chart of an opticalsemiconductor device used in the demultiplexer of FIG. 23 as a samplingdevice;

FIG. 25 is a diagram showing the relationship between the collectorcurrent and the input optical power of the optical semiconductor deviceof FIG. 24A;

FIG. 26 is a diagram showing the construction of the opticalsemiconductor device of FIG. 24A in a cross sectional view;

FIG. 27 is a diagram showing the operational characteristics of FIG. 25in an enlarged scale;

FIGS. 28A-28C are circuit diagrams showing the construction of anoptical semiconductor device according to a nineteenth embodiment of thepresent invention;

FIG. 29 is a diagram showing the construction of an opticalsemiconductor device according to a twentieth embodiment of the presentinvention in a cross sectional view;

FIG. 30 is a diagram showing the construction of an opticaldemultiplexer according to a twenty-first embodiment of the presentinvention;

FIG. 31 is a diagram showing the construction of an optical multiplexeraccording to a twenty-second embodiment of the present invention;

FIG. 32 is a diagram showing the construction of an opticalsemiconductor device according to a twenty-third embodiment of thepresent invention;

FIGS. 33A-33C are band diagrams showing the operation of the opticalsemiconductor device according to a twenty-fourth embodiment of thepresent invention;

FIGS. 34A-34C are waveform diagrams showing the operation of an opticalsemiconductor device according to a twenty-fifth embodiment of thepresent invention; and

FIG. 35 is a diagram showing the construction of an opticalsemiconductor device according to a twenty-sixth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows the principle of a light emitting device according to thepresent invention. As a more detailed description is given withreference to the embodiments to be described later, only a brief summaryof the operational principle of the device will be given here.

Referring to FIG. 2, the light emitting device has a structure similarto a multiple emitter bipolar transistor and includes a collector layer1 of a first conductivity type on which a base layer 2 of a second,opposite conductivity type is provided, and an emitter layer 3 of thefirst conductivity type is provided further on the base layer 2 as,usual in a bipolar transistor. The bipolar transistor of the instantinvention differs from the conventional bipolar transistors in that theemitter layer 3 includes a first emitter region 3₁ and a second,different emitter region 3₂ such that the region 3₂ is separated fromthe first region 3₁ by a thin intervening or bridging region 3₃. Itshould be noted that the bridging region 3₃ of the emitter layer 3prevents the base layer 2 from being exposed, and thus, the base layeris protected from damage during the fabrication process of the device aswell from contamination during the use of the device.

On the first and second emitter regions 3₁ and 3₂, first and secondemitter electrodes 5₁ and 5₂ are provided respectively, while acollector electrode 4 is provided on the lower major surface of thecollector layer 1. Thus, the light emitting device of FIG. 2 has astructure similar to that of a multiple emitter bipolar transistorproposed previously by the inventor in the U.S. Pat. No. 5,561,306(application Ser. No. 08/295,538), op cit, which is incorporated hereinas a 5 reference. It should be noted that the device of FIG. 2 includesno base electrode. Similarly to the device of the foregoing '306 patent('538 application), the emitter layer 3 is formed of a compoundsemiconductor material having a wide bandgap as compared with thesemiconductor material forming the base layer.

In operation, electrons and holes are injected into the base layer 2from the first emitter electrode 5₁ and the second emitter electrode 5₂,wherein the electrons and holes thus injected cause a recombination inthe base layer 2 to form an optical radiation as indicated in FIG. 2 byhν. The optical radiation thus formed is emitted from an edge surface ofthe base layer 2.

FIG. 3A shows the principle of a photodetection device according to thepresent invention. Again, only a brief summary about the principle ofthe device will be given here.

Referring to FIG. 3A, the device has a structure similar to that of FIG.2 and includes a collector layer 11 of a first conductivity type onwhich a base layer 12 of a second, opposite conductivity type isprovided, and an emitter layer 13 of the first conductivity type isprovided further on the base layer 12, as is usual in a bipolartransistor. Further, the emitter layer 13 includes a first emitterregion 13₁ and a second, different emitter region 13₂ such that theregion 13₂ is separated from the first region 13₁ by a thin interveningor bridging region 13₃. In this case, too, the bridging region 13₃protects the surface of the base layer 12 from damage. On the emitterregions 13₁ and 13₂, emitter electrodes 15₁ and 15₂ are providedrespectively, while a collector electrode 14 is provided on the lowermajor surface of the collector layer 11.

FIG. 3B shows the band structure of the device of FIG. 3A along a linepassing consecutively through the first electrode 15₁, the first emitterregion 13₁, the base layer 12, the second emitter region 13₂ and thesecond electrode 15₂, wherein the continuous line of FIG. 3B shows thestate wherein no optical energy is applied to the base layer 12. In thisstate, it will be noted in FIG. 3B that both the bottom edge of theconduction band and the top edge of the valence band are located at anenergy level higher than the energy level of any part of the bottom edgeof the conduction band and the top edge of the valence band of the firstand second emitter electrodes 15₁.

When an optical energy is applied to the base layer 12 in this statewith an energy exceeding the bandgap of the base layer 12, electrons andholes are excited optically, as is well known in the art, wherein theelectrons thus excited occupy the conduction band of the base layer 12while the holes occupy the valence band of the same base layer 12. Thus,by applying a bias voltage between the first emitter electrode 15₁ andthe second emitter electrode 15₂ with a magnitude set such that nosubstantial current flows therebetween in the absence of any opticalradiation, it will be noted that electrons excited to the conductionband escape immediately from the base layer 12 to the emitter region 13₂by causing a tunneling through the depletion region formed between thebase layer 12 and the emitter region 13₂. On the other hand, holes thatare excited simultaneously due to the electrons are accumulated in thebase layer 12. It should be noted that the base layer 12 has the topedge of the valence band at a level higher than the top edge of thevalence band of either of the first and second emitter regions 13₁ and13₂. As the holes are accumulated in the base layer 12 and do not reachthe emitter electrode 15₁, no current flows between the electrodes 15₁and 15₂.

When such an accumulation of the holes occurs in the base layer 12, onthe other hand, the energy level of the base layer 12 shifts in thedownward direction as indicated in FIG. 3B by a broken line. When thelevel of the valence band of the base layer 12 has lowered and becomeequal to the level of the valence band of the emitter regions 13₁ and13₂, the holes can now escape from the base layer 12 to the emitterregion 13₁. Thereby, a current flows between the emitter electrode 15₁and the emitter electrode 15₂. In other words, it is possible to detectthe optical radiation hν by detecting the current flowing between bothemitter electrodes 15₁ and 15₂.

Alternatively, one may bias the device such that a bias voltage isapplied between the first emitter electrode 15₁ and the collectorelectrode 14. Thereby, the electrons excited as a result of the opticalradiation applied to the base layer 12 escape immediately to thecollector electrode 14, leaving an accumulation of the holes in the baselayer 12.

One remarkable feature of the present invention, which is related to thelatter operational mode, is that, by providing the second emitter region13₂ as well as the second emitter electrode 15₂, one can injectelectrons from the second emitter region 13₂ to the base layer 12 so asto neutralize the excessive holes therein by causing a recombination.Thereby, the device resumes the initial state indicated by thecontinuous line in FIG. 3B immediately, whenever the optical radiationhν is interrupted. It should be noted that the injection of theelectrons from the second emitter region 13₂ to the base layer 12 isfacilitated by increasing the impurity concentration level of theemitter region 13₂. As a result of such an increased impurityconcentration level in the emitter region 13₂, the thickness of thedepletion region formed between the base layer 12 and the emitter layer13₂ is substantially reduced and the inter-band tunneling of theelectrons through such a depletion region is facilitated. In otherwords, the photodetection device of the present invention provides avery fast response.

First Embodiment!

FIG. 4 shows the construction of a light emitting device according to afirst embodiment of the present invention.

Referring to FIG. 4, the semiconductor device includes a collector layer21 of n-type InGaAs having a thickness of 300 nm and an impurityconcentration level of 3×10¹⁶ cm⁻³ on which a p-type base layer 22 ofInGaAs is provided. The base layer 22 may have a thickness of 80 nm andan impurity concentration level of 5×10¹⁹ cm⁻³. Further, an emitterlayer 23 of n-type InGaAs is provided further on the base layer 22 withan impurity concentration level of 3×10¹⁸ cm⁻³, wherein the emitterlayer 23 includes a first emitter region 23₁ and a second emitter region23₂ both having a thickness of 200 nm. Further, a collector electrode 24of Cr/Au is provided on the lower major surface of the collector layer21 with a thickness of 20 nm for the Cu layer and a thickness of 300 nmfor the Au layer. Similarly, first and second emitter electrodes 25₁ and25₂ are provided respectively on the first and second emitter regions23₁ and 23₂, wherein each of the emitter electrodes includes a stackingof a Cr layer having a thickness of 20 nm and a Au layer having athickness of 300 nm.

It should be noted that the base layer 22 is covered entirely by theemitter layer 23 by forming a bridging region 23₃ between the emitterregions 23₁ and 23₂. By doing so, the base layer 22 is protected fromdamage during the fabrication process of the device. Further, anysurface degradation during the use of the device is effectivelyeliminated.

In the device of FIG. 4, electrons and holes are injected to the baselayer 22 by applying a voltage across the first and second emitterelectrodes 25₁ and 25₂. The electrons and holes thus injected cause arecombination in the base layer 12, and in response to such arecombination, an optical radiation h occurs generally at the junctionregion between the base layer 22 and the emitter region 23₁ and at thejunction region between the base layer 22 and the emitter region 23₂,wherein the optical radiation thus formed is emitted from an edgesurface of the base layer 22. In order to facilitate optical radiation,one may provide anti-reflection coatings 22a and 22b of SiN or othersuitable material on the edge surfaces of the base layer 22. Thereby,the anti-reflection coatings 22a and 22b form an optical window.

In the device of FIG. 4, it should further be noted that one can reducethe operational voltage of the device by increasing the impurityconcentration level of the emitter regions 23₁ and 23₂ by about fivetimes as large as the concentration level used in a HBT. By doing so,the recombination of the electrons and holes at the foregoing junctionregions is facilitated and the device can show a laser oscillation whenmirrors are provided on both edge surfaces of the base layer 22.

Second Embodiment!

FIG. 5 shows the construction of a light emitting device according to asecond embodiment of the present invention. As the device of FIG. 5 hasa construction similar to that of FIG. 4, those parts corresponding tothe device of FIG. 5 are designated by the same reference numerals.

Referring to FIG. 5, the device includes a collector contact layer 21cof n-type InGaAs having a thickness of 300 nm and an impurityconcentration level of 5×10¹⁸ cm⁻³, and a collector layer 21 provided onthe collector contact layer 21c formed as such. Similarly as before, thecollector layer 21 is formed of n-type InAlAs and has a thickness of 300nm, wherein the layer 21 of the device of FIG. 5 is doped to an impurityconcentration level of 1×10¹⁷ cm⁻³. Further, the base layer 22 of p-typeInGaAs is provided on the collector layer 21 with a thickness of 70 nmand an impurity concentration level of 5×10¹⁹ cm⁻³ similarly as before,and the emitter layer 23 of n-type InAlAs, doped to a concentrationlevel of 3 ×10¹⁸ cm⁻³ is provided further on the base layer 22 with athickness of 200 nm. On the emitter layer 23, a first emitter contactregion 23c₁ of n-type InGaAs is provided with a thickness of 200 nm andan impurity concentration level of 5×10¹⁹ cm⁻³ in place of the region23₁ of FIG. 4. Similarly, a second emitter contact region 23c₂ is formedin place of the emitter contact region 23₂, wherein the emitter contactregion 23c₂ is formed of n-type InGaAs doped to the impurityconcentration level of 5×10¹⁹ cm⁻³ and has a thickness of 200 nm. On theregions 23c₁ and 23c₂, the emitter electrodes 25₁ and 25₂ are providedrespectively.

In this device, too, the electrons and holes cause, upon injection tothe base layer 22 from the emitter electrodes 25₁ and 25₂ respectively,a recombination at the areas of the base-emitter junction correspondingto the emitter contact region 23c₁ and the emitter contact region 23c₂.As a result of such a recombination, optical radiation is producedsimilarly as before. The optical radiation thus formed is emittedthrough the anti-reflection coatings 22a and 22b at the edges of thebase layer 22 that acts as an optical window.

Next, the fabrication process of the device of FIG. 5 will be describedwith reference to FIGS. 6A-6C. As will be noted, the device fabricatedby the instant process is slightly different from the device of FIG. 5in that the emitter electrode 24 is provided not on the lower majorsurface of the collector contact layer 21c but on the exposed uppermajor surface thereof.

Referring to FIG. 6A, the collector contact layer 21c of n-type InGaAsis deposited on a semi-insulating substrate of InP by a MBE process,followed by a MBE deposition of the collector layer 21 of n-type InAlAsconducted upon the collector contact layer 21c. Further, the depositionof the base layer 22 of p-type InGaAs is made on the collector layer 21by a MBE process, followed by a MBE deposition of the emitter layer 23of n-type InAlAs further on the base layer 22, and a emitter contactlayer 23c of n-type InGaAs is provided on the emitter layer 23 asindicated in FIG. 6A.

Next, in the step of FIG. 6B, a resist pattern (not shown) is providedon the emitter contact layer 23c so as to form an aperture exposing thesurface of the layer 23c at a predetermined area, and a wet etchingprocess is applied against the emitter contact layer 23c whileprotecting the surface of the layer 23 except for the part where theexposure aperture is formed. The etching process may be conducted byusing a phosphoric acid as an etchant. As a result of the etching, anemitter mesa structure is formed, wherein the emitter mesa structureincludes the emitter contact regions 23c₁ and 23c₂.

Next, as illustrated in FIG. 6C, a resist pattern is provided so as tocover the structure of FIG. 6C in correspondence to the part in whichthe bipolar transistor is to be formed, and the layers 23c through 21are removed successively by applying a wet etching process of aphosphoric etchant, until the collector contact layer 21c is exposed. Asa result, a collector mesa structure is formed as indicated in FIG. 6Cin which it will be noted that the upper major surface of the collectorcontact layer 23c is exposed outside the device region. After thecollector mesa is formed as such, the resist mask is removed and theemitter electrodes 25₁ and 25₂ are provided respectively on the emittercontact regions 23c₁ and 23c₂.

In the structure of FIG. 6C, it will be noted that the emitter layer 23covers the entire surface of the base layer 22. As the emitter layer 23has a bandgap substantially larger than the bandgap of the base layer22, the photons produced as a result of the recombination of thecarriers are effectively confined in the base layer 22 and the opticalloss of the base layer 22 is substantially reduced.

It should be noted that the foregoing fabrication process of FIGS. 6A-6Cis substantially the same as the fabrication process of a conventionalmultiple-emitter HBT. In other words, the light emitting device of thepresent invention can be fabricated easily by using the conventionalfacility for fabricating multiple-emitter HBTs.

In the HBT light emitting device of the structure of FIG. 5 or FIG. 6C,it should be noted that one can effectively suppress the unwanted riseof the collector current caused by impact ionization, by forming thecollector region 21 with a wide gap material. Further, by forming boththe collector layer 21 with a wide gap material in addition to theemitter layer 23, it is possible to confine the photons produced by thebase layer 22 two-dimensionally. Thereby, the transmission loss of theoptical beam through the base layer 22 is further reduced.

Third Embodiment!

FIG. 7 shows the construction of a light emitting device according to athird embodiment of the present invention. In FIG. 7, those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

In the embodiment of FIG. 7, it should be noted that the bridging region23₃ of the emitter layer carries thereon a dummy base electrode 26.Thereby, the dummy base electrode 26 prevents any external opticalradiation from penetrating into the channel layer 22, and any influenceof the surrounding optical radiation upon the photon emission in thedevice is successfully eliminated. Further, by providing one or more ofthe electrodes 25₁, 25₂ and 26 with a material that is transparent to aselective wavelength as an optical window, it is possible to extract theoptical radiation of the selected wavelength from such electrodes.

Fourth Embodiment!

FIG. 8 shows the construction of a photodetection device according to afourth embodiment of the present invention.

Referring to FIG. 8, the photodetection device includes a collectorlayer 31 of n-type InGaAs having a thickness of 300 nm and an impurityconcentration level of 3×10¹⁶ cm⁻³, on which a p-type base layer 32 ofInGaAs is provided. The base layer 32 may have a thickness of 80 nm andan impurity concentration level of 5×10¹⁹ cm⁻³. Further, an emitterlayer 33 of n-type InGaAs is provided further on the base layer 22 withan impurity concentration level of 3×10¹⁸ cm⁻³, wherein the emitterlayer 33 includes a first emitter region 33₁ and a second emitter region33₂ both having a thickness of 200 nm. Further, a collector electrode 34of Cr/Au is provided on the lower major surface of the collector layer31 with a thickness of 20 nm for the Cu layer and a thickness of 300 nmfor the Au layer. Similarly, first and second emitter electrodes 35₁ and35₂ are provided respectively on the first and second emitter regions33₁ and 33₂, wherein each of the emitter electrodes includes a stackingof a Cr layer having a thickness of 20 nm and a Au layer having athickness of 300 nm.

It should be noted that the base layer 32 is covered entirely by theemitter layer 33 by forming a bridging region 33₃ between the emitterregions 33₁ and 33₂. By doing so, the base layer 32 is protected fromdamage during the fabrication process of the device. Further, anysurface degradation during the use of the device is effectivelyeliminated.

In operation, an optical beam is injected to the base layer 32 with anoptical energy hν set so as to exceed a bandgap Eg of the base layer 32.In the illustrated example, the incident optical beam is injected viathe bridging region 33₃ connecting the first and second emitter regions33₁ and 33₂. Thereby, the bridging region 33₃ acts as an optical window.In response thereto, electron-hole pairs are excited in the base layer32, and the electrons thus excited occupy the conduction band of thebase layer 32 while the holes occupy the valence band. Thus, theelectrons on the conduction band are removed quickly, while the holes inthe valence band are not removed, and there occurs an accumulation ofholes in the base layer 32.

With the accumulation of the holes in the base layer 32, on the otherhand, the energy level of the base layer 32 shifts lower with respect tothe emitter regions 33₁ and 33₂ for both the conduction band and thevalence band, and the current starts to flow between the electrodes 35₁and 35₂ when the level of the valence band of the base layer 22coincides with the level of the valence band of the emitter regions 33₁and 33₂. In this case, both the electrons and holes contribute to thecurrent flowing between the electrodes 35₁ and 35₂.

Thus, by detecting the current between the emitter electrodes 35₁ and35₂, it is possible to detect the optical radiation incident to thedevice of FIG. 8. When operating the photodetection device according tothe operational mode as set forth above, the collector layer 31 is heldat a low potential level such that there is no substantial flow ofelectrons from the base layer 32 to the collector layer 31. By applyinga bias voltage between the electrodes 35₁ and 35₂ such that the biasvoltage does not exceed a predetermined threshold, it is possible toachieve a sharp rise of the output current in response to the opticalradiation.

Alternatively, the device of the present may be operated by applying abias voltage across one of the emitter electrodes such as the electrode35₁ and the collector electrode 34. The bias voltage is set to have amagnitude such that no substantial current flows from the emitterelectrode 35₁ to the collector electrode 34. In response to theradiation of the optical beam upon the base layer 32 as indicated inFIG. 8, the electrons and holes are excited similarly as before, and theelectrons thus excited are caused to flow to the collector layer 31 bythe electric field created by the collector bias voltage. On the otherhand, the holes remain in the base layer 32 because of the same reasonas before, and there occurs an accumulation of the holes.

In the device of FIG. 8, it is possible to neutralize or annihilate suchholes in the base layer 32 by injecting electrons thereinto from theother emitter electrode 35₂ via the second emitter region 33₂ such thatthe electrons thus injected cause a recombination with the holesaccumulated in the base layer 32. By doing so, the original operationalstate of the device is recovered immediately and the device is ready fordetecting a subsequent optical pulse. In other words, the photodetectionof the device of FIG. 8 shows an excellent response under such a biasingscheme. As there occurs a substantial current amplification in thedevice that has the structure of a bipolar transistor, the device showsa very large sensitivity due to the current amplification achievedinherently by the device.

Fifth Embodiment!

FIG. 9 shows the construction of a photodetection device according to afifth embodiment of the present invention. In FIG. 9, those partsdescribed previously with reference to FIG. 8 will be designated by thesame reference numeral and the description thereof will be omitted.

In the embodiment of FIG. 9, a dummy base electrode 36 is provided so asto cover the bridging region 33₃ of the emitter layer 33. Thereby, thedevice of FIG. 9 receives the incident optical beam that impinges intothe base layer 32 from an edge surface thereof as indicated in FIG. 9 byhν. On the other hand, the optical beam or light incident vertically tothe device as indicated in FIG. 9 by hν₁ is effectively interrupted. Inother words, the cross-talk of the optical beam hν₁ to the opticalsignals in the optical beam hν is effectively eliminated. Further, inthe device of FIG. 9, the response speed of the device is improvedfurther by supplying a current from the dummy base electrode 36 with amagnitude below a predetermined threshold. Of course, any opaque coveror film including insulators may be used for the dummy base electrode36, as long as the opaque cover is not used for an electrode.

As will be described later, one can construct a masked ROM by arrangingthe device of FIG. 9 in rows and columns, with the dummy base electrode36 provided only on selected devices in accordance with the informationto be stored in the ROM. Thereby, a large number of such masked ROMs areassembled together to form a memory device such that each masked ROMcarries a mask pattern pertinent thereto. By selectively hitting one ofthe masked ROMs in the memory device with an optical beam, it ispossible to read out the information stored in the selected masked ROM.

In the device of FIG. 9, one may provide an anti-reflection film 32a tocover the edge surface of the base layer 32.

Sixth Embodiment!

FIG. 10 shows the construction of a photodetection device according to asixth embodiment of the present invention. In FIG. 6, those partsdescribed previously are designated by the same reference numerals andthe description thereof will be omitted.

Referring to FIG. 10, the device has a structure similar to that of thedevice of FIG. 9 in that both devices include the dummy base electrode36. In the device of FIG. 10, the dummy base electrode 36 is actuallyformed of an optical filter such as a semiconductor or dielectricmultilayer filter that has an optical passband coincident to thewavelength of the incident optical beam hν. Thus, the device of FIG. 10is used to modulate the collector current selectively in response to theoptical input supplied with a wavelength that matches the opticalpassband of the filter 36. Further, one may provide an anti-reflectionfilm in place of the filter 36.

Similarly to the embodiment of FIG. 9, the device of FIG. 10 can be usedto construct a masked ROM by arranging the device of FIG. 10 in rows andcolumns, with the dummy base electrode 36 provided only on selecteddevices in accordance with the information to be stored in the ROM.Thereby, a large number of such masked ROMs may be assembled together toform a memory device such that each masked ROM carries a mask patternpertinent thereto. By selectively hitting one of the masked ROMs in thememory device with an optical beam, it is possible to read out theinformation stored in the selected masked ROM.

Seventh Embodiment!

FIG. 11 shows a photodetection device according to a seventh embodimentof the present invention. In FIG. 11, those parts corresponding to theparts described previously are designated by the same reference numeralsand the description thereof will be omitted.

In the present embodiment, the device has a structure similar to that ofthe device of FIG. 9 except that a collector contact layer 31c of n-typeInGaAs is provided with a thickness of 300 nm and an impurityconcentration level of 5×10¹⁸ cm⁻³, and the collector layer 31 isprovided on the collector contact layer 31c formed as such, wherein thecollector layer 31 has a composition of InGaAs and is doped to then-type with an impurity concentration level of 1×10¹⁷ cm⁻³. Thecollector layer 31 is provided with a thickness of 300 nm. On thecollector layer 31, the base layer 32 is provided with the compositionof InGaAs doped to the p-type similarly to the device of FIG. 9, with athickness of 70 nm and with an impurity concentration level of 5×10¹⁹cm⁻³. Further, the emitter layer 33 is provided on the base layer with acomposition of InAlAs, wherein the emitter layer 33 has a thickness of200 nm and is doped to the n-type with an impurity concentration levelof 3×10¹⁸ cm⁻³.

On the emitter layer 33, it is noted that the first and second emittercontact regions 33c₁ and 33c₂ are provided with a thickness of 200 nm,wherein each of the regions 33c₁ and 33c₂ is doped to an impurityconcentration level of 5×10¹⁹ cm⁻³. Further, the emitter contact regions33c₁ and 33c₂ carry thereon the emitter electrodes 35₁ and 35₂respectively, and the collector contact layer 31c carries the collectorelectrode 34 on the lower major surface thereof similarly as before,wherein the collector electrode 34 is formed of a stacking of a Cr layerhaving a thickness of 20 nm and an Au layer having a thickness of 300nm.

In the device of FIG. 11, it should be noted that the emitter electrodes35₁ and 35₂ are formed of a conductive material that is transparent to aselected wavelength of the optical radiation. For example, the emitterelectrodes 35₁ and 35₂ may be formed of a material such as InSnO orNdIn₂ O₄ that has a spinel structure. By using such transparentelectrodes, it is possible to inject optical radiation via the emitterelectrodes 35₁ and 35₂. Particularly, the latter material having thecomposition of NdIn₂ O₄ is suitable for the present purpose, as thematerial has a broad optical passband ranging from near infrared tovisible wavelengths and simultaneously shows an excellent conductivityin the order of 10⁴ S/cm.

Eighth Embodiment!

FIG. 12 shows the photodetection device according to an eighthembodiment of the present invention in a plan view.

Referring to FIG. 12, the device has a structure similar to that of FIG.11 except that the emitter electrodes 35₁ and 35₂ are eliminated and theemitter contact regions 33c₁ and 33c₂ are exposed. In place of theemitter electrodes 35₁ and 35₂, the device of FIG. 12 uses emitter leads37₁ and 37₂ such that the emitter leads 37₁ and 37₂ are connected tomarginal regions of the emitter contact regions 33c₁ and 33c₂,respectively. By eliminating the emitter electrodes 35₁ and 35₂, thedevice of FIG. 12 minimizes any optical loss of the optical beamincident to the emitter contact regions 33c₁ and 33c₂. In the device ofFIG. 12, the impurity concentration level of the emitter contact regions33c₁ and 33c₂ is increased for reducing the resistance thereof.

Ninth Embodiment!

FIG. 13 shows a photodetection device according to a ninth embodiment ofthe present invention. In FIG. 13, those parts corresponding to theparts described previously are designated by the same reference numeralsand the description thereof will be omitted.

In the device of FIG. 13, a Fabry-Perot resonator 38 is provided on thelower major surface of the collector layer 31, and the collectorelectrode 34 is provided on the lower major surface of the Fabry-Perotresonator 38 thus formed. The Fabry-Perot resonator is formed of analternating stacking of an InAlAs film and an InGaAlAs film repeatedeight times, wherein each of the InAlAs and InGaAlAs films has athickness corresponding to a quarter wavelength of the optical beam tobe detected.

By employing such a Fabry-Perot resonator 38, it is possible induce aresonance in the incident optical beam with an optical beam reflected bythe Fabry-Perot resonator 38, such that the anti-node of resonance isformed coincident to the base layer 32 or the collector layer 31.Thereby, the efficiency of photoelectric conversion is substantiallyimproved.

Tenth Embodiment!

Next, the photodetection device according to a tenth embodiment of thepresent invention will be described with reference to FIGS. 14A-14Eshowing a band diagram of the device. In the drawings, the vertical axisrepresents the energy level as usual in a band diagram. As the device ofthe present embodiment has a structure of any of the previouslydescribed devices, further description about the structural feature ofthe device will be omitted. In the description hereinafter, the emitterregion 33₁ or the emitter contact region 33c₁ is designated as a firstemitter region E₁, the emitter region 33₂ or the emitter contact region33c₂ is designated as a second emitter region E₂, the base layer 32 isdesignated as a base region B, and the collector layer 31 is designatedas a collector region C.

Referring to FIG. 14A, it will be noted that the first emitter region E₁and the second emitter region E are biased to respective positive biasvoltages V_(E1) and V_(E2), wherein the voltage V_(E1) is setsubstantially higher than the voltage V_(E2) in the state of FIG. 14A(0<V_(E1) <<V_(E2)). It should be noted that no optical radiation isapplied to the device in the state of FIG. 14A. As there occurs nocreation of electrons e and holes h in the base layer B in the absenceof the optical radiation, no collector current flows through the device.

In the state of FIG. 14B, on the other hand, an optical radiation hν isapplied to the base layer B while maintaining the device in the samebiasing state as in the case of FIG. 14A. Thereby, electrons e and holesh are created in the base region B as a result of optical excitation,and the electrons thus created immediately escape to the collectorregion C along the potential slope of the conduction band. In otherwords, there flows a collector current Ic. On the other hand, the holesdwell in the base region B that forms a potential well against holes,and there occurs an accumulation of the holes in the base region B as aresult.

With further continuation of the optical beam radiation, theaccumulation of the holes in the base region B continues, and the energylevel of the base region B shifts lower as a result of such holeaccumulation. Such a lowering of the energy level of the base region B,in turn, allows the electrons in the first emitter region E₁ to flowinto the base region B as indicated in FIG. 14D and further into thecollector region C, and the collector current Ic flows in response tothe flow of the electrons.

FIG. 14E shows the state in which the optical radiation is interruptedand the voltage V_(E2) of the emitter region E₂ is slightly lowered to alevel still higher than the voltage V_(E1) of the emitter region E₁(0<V_(E1) <E₂). In this state, the electrons are injected from thesecond emitter region E₂ into the base region B to cause an annihilationof holes accumulated therein. Upon annihilation of the holes in the baseregion B, the energy level of the base region B increases again to theinitial state of FIG. 14A. In response to this, the injection of theelectrons from the emitter region E₁ to the base region B is effectivelyinterrupted, and hence the collector current Ic.

In order to facilitate such an injection of the electrons from theemitter region E₂ to the base region B, the device of the presentinvention has a construction such that the base region B and the emitterregions E₁ and E₂ are doped with respective impurity concentrationlevels such that the impurity concentration level of the base region Bexceeds the effective density of state of the semiconductor materialforming the base region B and such that the impurity concentration levelof the emitter regions E₁ and E₂ exceeds the effective density of stateof the semiconductor material forming the emitter regions. Thereby, thep-n junction between the base region B and the emitter region E₂ easilycauses a breakdown and the electrons are injected efficiently from theemitter region E₂ to the base region B by causing a tunneling throughthe depletion region associated with the p-n junction. By externallyinjecting the electrons to the base region B, one can reduce therecovery time of the device and the photodetection device shows anexcellent response against high speed optical input.

Eleventh Embodiment!

A similar operational principle described with reference to the tenthembodiment of the present invention for improved response, is applicablealso to a photodetection device having the structure of a single-emitterHBT, by doping the emitter region and the base region to the impurityconcentration levels exceeding the respective effective density ofstates.

FIGS. 15A-15C show the operation of a conventional HBT when used for aphotodetection device, wherein FIGS. 15A-15C show the band structure ofthe device similarly to the band diagrams of FIGS. 14A-14E. In thedrawings, the emitter layer of the device is represented by an emitterregion E, the base layer by a base region B, and the collector layer bya collector region C.

Referring to FIG. 15A, the device is biased such that a negative voltageis applied to the emitter region E and a positive voltage to thecollector region C. On the other hand, no optical radiation is appliedto the base region B. In this state where the base region B forms apotential barrier, no excitation of electrons and holes occurs, and nocollector current Ic appears.

In the state of FIG. 15B, on the other hand, an optical radiation isapplied to the base region B while holding the voltage level of theemitter region E and the collector region C similarly as in the case ofFIG. 15A. In this state, there occurs a creation of electrons and holesin the base region B, while the electrons thus created escapeimmediately to the collector region C along the slope of the conductionband, leaving the holes in the base region B. As a result of such anaccumulation of the holes, the energy level of the base region Bdecreases with respect to the emitter region E, and the injection ofelectrons from the conduction band of the emitter region E to theconduction band of the base region B starts. The electrons thus injectedimmediately escape to the collector region C by drifting along thesloped conduction band between the base region B and the emitter regionC, and a collector current Ic starts to flow.

FIG. 15C shows the state in which the irradiation of the optical beam isinterrupted in the state of FIG. 15B. In the state of FIG. 15C, it willbe noted that the accumulation of the holes in the base region B remainsand hence the collector current Ic keeps on flowing. Until the holes inthe base region B are dissipated, the device does not recover theoriginal state of FIG. 15A. In other words, the conventionalphotodetection devices having the structure of a bipolar transistor havesuffered from the problem of slow response.

In the present embodiment, the foregoing problem of slow recovery of theinitial state of the device is successfully eliminated by increasing theimpurity concentration level in the emitter region E and the base regionB such that impurity concentration level exceeds the effective densityof state in each of the emitter region E and the base region B. Byincreasing the impurity concentration level in the emitter and baseregions of a bipolar transistor, it is possible to reduce the thicknessof the depletion region formed at the p-n junction between the baseregion B and the emitter region E.

Thus, upon irradiation of the optical beam on the base region B, theenergy level of the conduction band and the valence band of the baseregion B decreases as indicated in FIG. 16B, and the electrons in theemitter region start to flow from the emitter region E to the collectorregion C via the base region B, along the conduction band similarly asin the case of FIG. 15B. Thereby, a collector current Ic flows from thecollector C to the emitter E.

In the step of FIG. 16C where the irradiation of the optical beam isinterrupted, it should be noted that the electrons on the conductionband of the highly doped emitter region E cause a tunneling to thevalence band of the base region B through the depletion layer formed atthe emitter-base junction, because of the reduced depletion layerthickness. Thereby, the holes accumulated in the base region B arequickly neutralized, and the device resumes the initial band structureof FIG. 16A.

Twelfth Embodiment!

FIG. 17 shows the construction of a semiconductor optical switchaccording to a twelfth embodiment of the present invention. In FIG. 17,those parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

Referring to FIG. 17, the optical switch has a structure similar to thatof FIG. 8 except that the device includes the collector contact layer31c underneath the collector layer 31, wherein the collector contactlayer 31c is provided upon a semi-insulating substrate 30 of InP.

In the device of FIG. 17, it should be noted that an inactive region 39is formed in the collector layer 31 as well as in the collector contactlayer 31c underneath thereof by an ion implantation process, such thatthe inactive region 39 extends from the collector contact layer 31c upto the collector layer 31. Thereby, the collector layer 31 is laterallydivided into a first collector region 31₁ and a second collector region31₂ by the inactive region 39. Similarly, the collector contact layer31c is divided into a first region 31c₁ and a second region 31c₂. Itshould be noted that the inactive region 39 does not penetrate into thebase layer 32. Further, in correspondence to the first and secondcollector regions 31c₁ and 31c₂, there are provided first and secondcollector electrodes 34₁ and 34₂.

It should be noted that the device of FIG. 17 may employ aheterojunction of the InGaAs/InAlGaAs system for the base-emitterjunction. In this case, the base layer 32 is formed of InGaAs doped tothe p-type, while the emitter layer 33 is formed of n-type InAlGaAs.Further, the collector layer 31 is formed of n-type InGaAs, not ofInAlGaAs. The doping of such a InGaAs/InAlGaAs system is preferablyconducted by using Be or C for the p-type dopant.

In operation, electrons and holes are injected to the base layer 32 fromthe emitter electrodes 35₁ and 35₂, such that photon emission in thebase layer 32 occurs as a result of recombination. The optical radiationthus produced reaches the collector regions 31₁ and 31₂ in the collectorlayer 32, and induces a modulation of the collector current flowingbetween the foregoing first and second collector electrodes 34₁ and 34₂.In other words, one can switch the collector current in response to theoptical radiation induced in the base layer 32.

Thirteenth Embodiment!

Next, a semiconductor optical switch according to a thirteenthembodiment of the present invention will be described with reference toFIG. 18.

Referring to FIG. 18, the semiconductor optical switch is constructedupon a substrate of semi-insulating InP and includes a buffer layer 41of n-type InGaAs, a collector contact layer 42c of n-type InGaAs, acollector layer 42 of n-type InAlAs, a base layer 43 of p-type InGaAs,and an emitter layer 44 of n-type InAlAs, wherein the layers 41-44 aredeposited consecutively on the substrate 40 by an epitaxial process.

After the layered structure of FIG. 18 is formed, an anisotropic etchingprocess is applied to form mesa structures OE₁, OE₂ and EO₁ commonly onthe substrate 40, such that the mesa structures are isolated from eachother by an isolation trench reaching the surface of the substrate. Eachof the mesa structures OE₁, OE₂ and EO₁ has a structure similar to themultiple emitter device described previously and includes a pair ofemitter electrodes E. On the other hand, no base electrode is provided.Further, the mesa structures OE₁ and OE₂ includes a collector electrodeC in contact with the collector contact layer 42c. Thereby, a lightemitting device is formed in correspondence to the mesa structure EO₁,and photodetection devices are formed in correspondence to the mesastructures OE₁ and OE₂. It should be noted that the light emittingdevice EO₁ may have the structure of any of the first through thirdembodiments. On the other hand, the photodetection devices OE₁ and OE₂may have the structure of any of the fourth through tenth embodiments.

In operation, the photodetection device OE₁ is operated so as to detectthe incident optical beam by detecting the change of the current flowingfrom the first emitter electrode to the second emitter electrode. On theother hand, the photodetection device is operated so as to detect theincident optical beam by detecting the change of the current flowingfrom the collector electrode to one of the emitter electrodes. In any ofthese photodetection devices OE₁ and OE₂, there occurs an opticalemission in the light emitting device EO₁ and the photodetection devicesOE₁ and OE₂ cause a switching in response thereto. It should be notedthat the base layer is provided at the common level or height in each ofthe devices OE₁, OE₂ and EO₁. Thus, one can achieve an inherent opticalalignment between these devices, and the optical beam produced by thelight emitting device EO₁ is received by the photodetection device OE₁and OE₂ with reliability, without conducting any complex opticalalignment process.

Fourteenth Embodiment!

FIG. 19 shows the construction of a semiconductor optical integratedcircuit according to a fourteenth embodiment of the present invention.In FIG. 19, those parts corresponding to the parts described previouslyare designated by the same reference numerals and the descriptionthereof will be omitted.

In the present embodiment, the emitter layer 44 alone is subjected to anetching process to expose the upper major surface of the base layer 43,such that the devices OE₁, OE₂ and EO₁ are formed as emitter structuresthat are isolated from each other on the base layer 43.

In the present embodiment, the optical radiation produced by the lightemitting device EO₁ in the base layer 43 is guided along the base layer43 and reaches the photodetection devices OE₁ and OE₂, thereby causing aswitching of a collector current therein. Preferably, the emitter layer44 and the collector layer 42 are formed of an n-type wide gap materialsuch as InAlAs, such that an efficient optical confinement occurs in thebase layer 43. Further, one can minimize the optical loss by eliminatingthe edge surface in the devices OE₁, OE₂ and EO₁. It should be notedthat such an edge surface acts as a half transparent mirror and reflectsa part of the optical beam incident to the device.

Fifteenth Embodiment!

FIG. 20 shows the construction of a semiconductor optical integratedcircuit according to a fifteenth embodiment of the present invention. InFIG. 20, those parts described previously with reference to previousembodiments are designated by the same reference numerals and thedescription thereof will be omitted.

In the present embodiment, it is noted that the device has a structuresimilar to that of FIG. 19 except that there are formed isolationregions 45 between the devices OE₁, OE₂ and EO₁ by an ion implantationprocess of a deep impurity element such as oxygen, such that theisolation region 45 extends from the collector contact layer 42c to thebase layer 43. By providing the isolation regions 45 as such, it ispossible to minimize the interference in the operation of the devicesOE₁, OE₂ and EO₁.

Sixteenth Embodiment!

FIG. 21 shows the construction of a semiconductor optical integratedcircuit according to a sixteenth embodiment of the present invention. InFIG. 21, those parts described previously with reference to previousembodiments are designated by the same reference numerals and thedescription thereof will be omitted.

Referring to FIG. 21, it is noted that the device has a structuresimilar to that of the device of FIG. 20, except that there is providedan HBT of the multiple emitter construction adjacent to the lightemitting device EO₁ with an isolation trench formed between the HBT andthe device EO₁. The isolation trench thereby reaches the collectorcontact layer 42c and exposes the upper major surface thereof, and theisolation region 45 is formed in such an isolation trench to extend fromthe exposed upper major surface of the collector contact layer 42c tothe substrate 40.

By providing such a multiple emitter HBT commonly on the substrate ofthe optical semiconductor device, it is possible to drive the lightemitting device EO₁ easily. Such a multiple emitter HBT is suitable forforming an integrated circuit with the optical semiconductor devicesOE₁, OE₂ and EO₁, as the HBT has a common layered structure to thedevices OE₁, OE₂ and EO₁.

Seventeenth Embodiment!

FIG. 22 shows the construction of a semiconductor optical integratedcircuit according to a seventeenth embodiment of the present invention.In FIG. 22, those parts described previously with reference to precedingembodiments are designated by the same reference numerals and thedescription thereof will be omitted.

Referring to FIG. 22, the semiconductor optical integrated circuit has astructure similar to that of FIG. 21, except that there is furtherprovided an optical switch OSW described previously with reference toFIG. 17, such that the optical switch OSW is located adjacent to theHBT. As the optical switch OSW has a layered structure identical tothose of the devices OE₁, OE₂, EO₁ and HBT, the optical integratedcircuit is fabricated easily.

Eighteenth Embodiment!

Next, an electro-optical logic device according to an eighteenthembodiment of the present invention will be described with reference toFIG. 23. The electro-optical logic device may for example be used for anoptical demultiplexer that extracts an information signal of a desiredchannel from a time-divisional multiplex signal.

Referring to FIG. 23, there is provided an optical transmitter 51 thattransmits an optical signal to an optical signal demultiplexer 53 via anoptical fiber 52 in the form of time-divisional multiplex optical signal54, wherein the optical signal demultiplexer 53 is supplied with anelectric clock pulse 55 and carries out a demultiplexing of the signal54 in response thereto.

More specifically, the demultiplexer 53 converts the input opticalsignal 54 to an electrical signal and samples the same in response tothe clock pulse 54. The signals thus sampled are then supplied torespective output channels in the form of output signals 56. Thus, suchan optical demultiplexer 53 generally includes a photoelectricconversion device and a sampling circuit or demultiplexing circuit thatcarries out a time-divisional demultiplexing of the electric signalconverted from the optical signal by the photoelectric conversiondevice.

Conventionally, various optical semiconductor devices have been used forthe photoelectric conversion device, including PIN photodiodes,avalanche photodiodes and phototransistors. However, such conventionalphoto semiconductor devices have various shortcomings. For example, aPIN photodiode lacks the function of amplifier and requires acooperating preamplifier in the following stage. On the other hand, anavalanche photodiode requires high operational voltage in the order of25 volts and tends to introduce noise in the electric output signals.Further, a photodiode, which has a structure of a bipolar transistorequipped with a floating base, has a problem of a slow response due tothe accumulation of carries in the base.

In the present embodiment, therefore, the optical semiconductordemultiplexer 53 uses the photodetection device described in any of thefourth through tenth embodiments of the present invention, for the logicdevice that carries out the sampling of the optical signal. By usingsuch a photodetection device having the construction of multiple emitterbipolar transistor in the operational mode of the tenth embodimentdescribed with reference to FIGS. 14A-14E, a very high speed response isattained for the photodetection and sampling. Further, as noted in thedescription of the tenth embodiment, such a photodetection deviceproduces the electric output with electric amplification already appliedthereto.

FIG. 24A shows the equivalent circuit diagram of the photodetectiondevice of any of the foregoing fourth through tenth embodiments of thepresent invention as used for a sampling circuit of the opticaldemultiplexer 53 of FIG. 23.

Referring to FIG. 24A, it will be noted that the first emitter region E₁of the device is grounded, and a bias voltage Vcc is supplied to thecollector C of the device via a resistance R set to have a value of 4kΩ, for example. Further, the photodetection device is supplied with theelectric clock signal 53 at the second emitter region E₂ as designatedby IN₂ and further with the incident optical signal 54 at the baseregion B of the device as designated by IN₁. Thereby, the output of thedevice is obtained at the node where the resistance R and the collectorC are connected. The incident optical signal of course has an opticalenergy sufficient to cause excitation of electrons and holes in the baseregion B.

FIG. 24B shows the operation of the device of FIG. 24A.

Referring to FIG. 24B, it will be noted that the device of FIG. 24Aproduces an output in response to the optical input IN₁, in the form ofa voltage drop across the resistance R, wherein such a voltage dropoccurs only in the high level or positive interval of the electric clocksignal IN₂. During the low level interval of the clock signal IN₂, nosuch a voltage drop occurs.

The result of FIG. 24B indicates that one can sample the optical signalIN₁ in response to the electric clock signal IN₂. Further, the result ofFIG. 24B indicates that a sampling of the electric signal IN₂ ispossible by the optical clock pulse IN₁. Thereby, it should be notedthat the device of FIG. 24A operates as an electro-optical AND gate thatproduces a logic product of the input optical logic signal IN₁ and theinput electric logic signal IN₂. As already noted with reference to thetenth embodiment of the present invention, the device of FIG. 24A showsan extremely sharp response.

In FIG. 24B, it will be noted that there occurs a small voltage drop inthe output of the device of FIG. 24A in response to the high level stateof the electric clock signal IN₂, even when the input optical signal IN₁is interrupted. The reason of this will be described later withreference to FIG. 25 which shows a relationship between the collectorcurrent and the input optical power for a multiple emitterphotodetection device shown in FIG. 26.

Thus, before explaining FIG. 25, the structure of the semiconductordevice used for the sampling device of FIG. 24A will be describedbriefly.

Referring to FIG. 26 first, the device has a structure substantiallyidentical to the structure of the photodetection devices describedheretofore and is constructed upon a semi-insulating InP substrate 61,wherein the device includes a collector contact layer 62 of n-typeInGaAs provided on the substrate 61 with a thickness of 300 nm and animpurity concentration level of 5×10¹⁸ cm⁻³, a collector layer 63 ofn-type InGaAs provided on the collector contact layer 62 with athickness of 300 nm and an impurity concentration level of 1×10¹⁷ cm⁻³and a base layer 64 of p-type InGaAs provided on the collector layer 63with a thickness of 80 nm and an impurity concentration level of 5×10¹⁹cm⁻³. On the base layer 64, a first emitter region 65₁ and a secondemitter region 65₂ are formed on respective first and second regionsdefined on the base layer 64, wherein each of the emitter regions 65₁and 65₂ is formed of n-type InP with a thickness of 150 nm and has animpurity concentration level of 1×10¹⁹ cm⁻³. On the emitter regions 65₁and 65₂, emitter contact regions 66₁ and 66₂ both of n-type InGaAs areprovided respectively, wherein each of the regions 66₁ and 66₂ has athickness of 200 nm and is doped to an impurity concentration level of5×10¹⁹ cm⁻³. On the emitter contact regions 66₁ and 66₂, emitterelectrodes 67₁ and 67₂ are formed as the first and second emitterelectrodes E₁ and E₂.

Referring now to FIG. 25, the horizontal axis represents the opticalpower P₀ of the incident optical beam and the vertical axis representsthe collector current, wherein the illustrated example shows the casewhere the wavelength of the incident optical beam is set to 1.55 μm. InFIG. 25, it should be noted that the relationship between collectorcurrent and the input optical power is measured for various voltagesV_(EE) applied between the first and second emitter electrodes E₁ andE₂.

As will be noted in the relationship of FIG. 25, the collector currentIc increases with increasing input optical power, similar to aconventional phototransistor. On the other hand, FIG. 25 shows clearlythe feature pertinent to the multiple emitter, open-base photodetectiondevice in that the collector current Ic flows also when the inputoptical power is zero, if a large voltage is applied between bothemitters E₁ and E₂ as the voltage V_(EE). For example, it will be notedfrom FIG. 25 that the collector current Ic for the case in which thevoltage V_(EE) is set to 0.7 V, is about 300 μA larger than thecollector current Ic for the case in which the voltage V_(EE) is set to0 V, provided that no optical input is applied. On the other hand, thecollector current difference between the case in which the voltageV_(EE) is set to 0.7 V and the case in which the voltage V_(EE) is setto 0 V, reaches a value as large as 900 μA when an input optical powerof 420 μW is applied to the device.

Thus, by utilizing the foregoing feature, it is possible to construct anelectro-optical AND gate that performs a logic product operation betweenan optical input signal and an electric input signal. For example, oneobtains an output collector current Ic of 1200 μA in response to thevoltage V_(EE) of 0.7 V corresponding to the high level state of theinput optical logic signal and in response to the input electric powerof 420 μW corresponding to the high level state of the high level stateof the input electric logic signal. When the optical input signal aloneis turned off, on the other hand, the output collector current Ic of 300μA is obtained.

FIG. 27 shows the relationship similar to the one shown in FIG. 25 in anenlarged scale for the region of FIG. 25 where the input optical poweris small. Similarly as before, the horizontal axis represents the inputoptical power P₀ while the vertical axis represents the collectorcurrent Ic. The wavelength of the optical beam is set to 1.55 μm. InFIG. 27, the voltage V_(EE) is set variously to 0 V, 0.1 V, 0.2 V, 0.3V, 0.4 V and 0.5 V. In FIG. 27, it will be noted that the deviceoperates with a very low input optical power such as 22 μW, providedthat the voltage V_(EE) is set to about 0.5 V, which voltage iscomparable to the turn on voltage of the collector current of themultiple emitter HBT.

In the sampling device of the present embodiment for use in the multipleemitter HBT construction of the present embodiment, it will be notedthat a certain amount of collector current cannot be avoided when onlyone of the optical input signal and the electric input signal issupplied to the device. In other words, there occurs an inevitabledifference or offset in the collector current between the state in whichno optical or electric signals are supplied to the device and the statein which only one of the electric signal and the optical signal issupplied to the device. Such an offset is observed in the outputwaveform of the device shown in FIG. 24B. While such an offset in theoutput collector current may not influence the logic operation of thedevice, there may be a case in which such an offset in the outputcollector current should be avoided.

In order to avoid the occurrence of such an offset in the outputcollector current Ic, one may set the voltage V_(EE) smaller than thevoltage in which the collector current starts to flow, such as 0.5 V.For example, by setting the voltage V_(EE) to 0.4 V, one may suppressthe foregoing offset of the collector current to be less than 1.3 μA inthe absence of the input optical signal. In this state, no substantialoutput current flows through the device. Thereby, the optical power ofthe input optical signal is set also such that no substantial collectorcurrent flows when the optical signal alone is supplied.

In the present embodiment, it will be noted that one can construct anelectro-optical OR gate by the device of FIG. 26 by setting the voltageV_(EE) to a sufficiently high level such as 0.7 V and further by settingthe input optical power to a sufficiently high level such as 420 μW,such that the device causes a conduction when any one of the opticalinput signal and the electric input signal is supplied.

Nineteenth Embodiment!

FIG. 28A shows the construction of an optical logic circuit that usesthe optical semiconductor device having the open-base multiple emitterHBT construction described previously, wherein the optical logic circuitof the present embodiment addresses the problem of the offset in thecollector current Ic mentioned previously.

Referring to FIG. 28A, the logic circuit includes, in addition to theoptical semiconductor device designated as HBT₁, anothermultiple-emitter open-base HBT designated as HBT₂ having a structureidentical to the structure of the HBT₁, wherein each of the HBT₁ andHBT₂ has the collector C connected to the ground via a resistor R.Further, the emitter E₁ of the HBT₁ and the emitter E₁ of the HBT₂ areconnected commonly to a power supply V_(ES) via a common current sourceI₀. Thereby, the second emitter E₂ of the HBT₁ used for the opticalsemiconductor device, is supplied with the electric logic signal, whileno input is supplied to the second emitter E₂ of the HBT₂.

In operation, the input optical beam is supplied simultaneously to thebase B of both of the HBT₁ and HBT₂, and the output of the logic circuitis obtained as a difference between the collector voltage of the HBT₁obtained at an output terminal OUT₁ and the collector voltage of theHBT₂ obtained at an output terminal OUT₂. Thus, when an electric logicsignal is supplied to the emitter E₂ of the HBT₁ in addition to theoptical logic signal supplied to both of the HBT₁ and HBT₂, the HBT₁causes a conduction and a voltage drop appears across the outputterminals OUT₁ and OUT₂ as an output of the logic circuit.

On the other hand, when the electric signal alone is supplied to theemitter E₂ of the HBT₁ without an optical signal, little collectorcurrent flows through the device HBT₁, provided that the magnitude ofthe electric signal is set smaller than the threshold voltage at whichthe collector current starts to flow. Thereby, no substantial voltagedrop occurs at the collector C of the HBT₁, and the output of theoptical logic circuit is held zero.

Further, when an optical input is supplied to the base B of both of theHBT₁ and HBT₂ without the electric input signal supplied to the emitterE₂ of the HBT₁, substantially the same collector current flows throughboth of the HBT₁ and HBT₂, and no voltage drop appears across the outputterminals OUT₁ and OUT₂.

Of course, the voltage drop across the output terminal OUT₁ and OUT₂ iszero when there is no optical input and simultaneously there is noelectric signal to the emitter E₂ of the HBT₁.

FIG. 28B shows a modification of the circuit of FIG. 28A, wherein thefirst and second emitters E₁ and E₂ of the HBT₂ are shorted.

FIG. 28C shows another modification in which a single emitter HBT of aconventional construction is used for the HBT₂.

In any of the embodiments of FIGS. 28B and 28C, one can obtain anoperation similar to that of the circuit of FIG. 28A.

Twentieth Embodiment!

FIG. 29 shows a semiconductor device according to a twentieth embodimentof the present invention corresponding to the equivalent circuit diagramof FIG. 28C in which a single emitter HBT is used for the HBT₂.

Referring to FIG. 29, the device is constructed upon a semi-insulatingInP substrate 71 that carries thereon a collector contact layer 72 of n⁺-type InGaAs. On the collector contact layer 72, there is provided acollector layer 73 of n-type InGaAs, and a base layer 74 of p-typeInGaAs is provided on the collector layer 73. Further, an emitter layer75 of n-type InP is provided on the base layer 74, and an emittercontact layer 76 of n-type InGaAs is provided on the emitter layer 75.Further, an electrode layer 77 is provided in ohmic contact with theunderlying emitter contact layer 76.

The layered structure thus formed including the layers 72-77 is furtherdivided into a first part corresponding to the HBT₁ and a second partcorresponding to the HBT₂ (both of FIG. 28C) by an isolation trench thatexposes the surface of the substrate 71, wherein the HBT₁ is constructedupon a collector contact layer 72₁ forming a part of the collectorcontact layer while the HBT₂ is constructed upon a collector contactlayer 72₂ forming another part of the collector contact layer 72.Further, the layers 75-77 are subjected to an etching process to form agroove in the part corresponding to the HBT₁ such that the groove thusformed separates the first emitter structure E₁ from the second emitterstructure E₂. It should be noted that the first emitter structure E₁includes a first emitter region 75₁, a first emitter contact region 76₁and a first emitter electrode 77₁ respectively corresponding to theemitter layer 75, the emitter contact layer 76 and the emitter electrode77. Similarly, the second emitter structure E₂ includes a second emitterregion 75₂, a second emitter contact region 76₂ and a second emitterelectrode 77₂.

On the other hand, the HBT₂ includes a collector region 73₃, a baseregion 74₃, an emitter region 75₃, an emitter contact region 76₃ and anemitter electrode 77₃ respectively corresponding to the foregoingcollector layer 73, the base layer 74, the emitter layer 75, the emittercontact layer 76 and the emitter electrode 77. Further, the surface ofthe collector contact layer 72₁ is exposed in the HBT₁ and a collectorelectrode 78₁ is provided thereon. Similarly, the surface of thecollector contact layer 72₃ is exposed and a collector electrode 78₃ isprovided.

Thus, the HBT₁ forms a multiple-emitter HBT while the HBT₂ forms asingle emitter HBT. In other words, the structure of FIG. 29 correspondsto the equivalent circuit diagram of FIG. 28C. Thus, device of FIG. 29successfully eliminates the problem of offset collector current.

Twenty-first Embodiment!

FIG. 30 shows the construction of a demultiplexer for extracting signalsof various channels from a time-divisional multiplex optical signal inwhich the photodetection device described in any of the precedingembodiments is used.

Referring to FIG. 30, the device has a structure similar to that of FIG.26 except that a number of the devices HBT₁ -HBT₄ are provided on acommon substrate 81 in correspondence to the number of the channels CH₁-CH₄. In each of the devices, the first emitter E₁ is grounded and theother emitter E₂ is supplied with a timing pulse in the form of electricsignals. Further, an input optical beam is supplied to each of thedevices from a common optical source or transmitter designated by TX viaan optical waveguide. As each of the devices HBT₁ -HBT₄ has a structuredescribed already with reference to FIG. 26, for example, furtherdescription about the structure of the device will be omitted.

In the device of FIG. 30, the optical signal is sampled in each of thedevices HBT₁ -HBT₄ in response to the timing pulse supplied thereto, andthe outputs of the respective devices are obtained at respectivecollectors C provided in correspondence to the channel CH₁ -CH₄.

In the device of FIG. 30, it should be noted that one can alsodemultiplex the electric signals supplied to each of the devices HBT₁-HBT₄ in response to an optical timing signal.

Twenty-second Embodiment!

FIG. 31 shows the construction of a multiplexer for multiplexing opticalsignals of various channels into a single time-divisional multiplexelectric signal.

Referring to FIG. 31, there are provided optical signal sources 1-3 incorrespondence to the channels CH₁ -CH₃, wherein each of the opticalsignal sources 1-3 supplies an optical signal to a correspondingsampling device designated as HBT₁ -HBT₃. It should be noted that eachof the devices HBT₁ -HBT₃ has the construction described previously forexample with reference to FIG. 26 and has the first emitter E₁ connectedto the ground.

As will be noted in FIG. 31, the optical signals of the channels CH₁-CH₃ hit the base layer of the corresponding devices HBT₁ -HBT₃perpendicularly via respective waveguides, while the devices HBT₁ -HBT₃receive the timing pulses at the respective second emitters E₂ with atiming different in each channel. In response to the timing pulse, eachof the devices HBT₁ -HBT₃ produces an electric output indicative of theoptical signal sampled with the timing of the timing pulse at thecollector C, wherein the devices HBT₁ -HBT₃ have the respectivecollectors C connected commonly with each other. Thereby, atime-divisional multiplex signal is obtained on the output lineconnected to the foregoing collectors C. Such a time-divisionalmultiplex signal may for example be used for driving a layer diode toproduce a time-divisional multiplex optical signal.

Twenty-third Embodiment!

In any of the foregoing embodiments, it should be noted that the opticalbeam applied to the device generally has a diameter of 100 μm or more.Further, it is generally practiced to secure a photoreception area of100 μm² in order to achieve a satisfactory optical alignment between theoptical fiber and the optical semiconductor device. On the other hand,such a construction has a drawback of increased base resistance due tothe increased distance between the first emitter region E₁ and thesecond emitter region E₂.

The present embodiment eliminates the foregoing problem by forming theemitter regions from a number of small emitter elements as indicated inFIG. 32.

Referring to FIG. 32, the optical semiconductor device is constructedupon a substrate 91 that carries thereon a collector contact layer 92,wherein a collector layer 93 is provided on the collector contact layer92 and a base layer 94 is provided on the collector layer 93. Further, anumber of emitter regions 35₁ -35₇ are provided on the base layer 94,leaving the exposed surface of the base layer between adjacent emitterregions, and emitter electrodes 36₁ -36₇ are provided on the emitterregions 35₁ -35₇ respectively. Thereby, the emitter regions 35₁ -35₇ andthe emitter electrodes 36₁ -36₇ thereon form a plurality of emitterregions E₁ -E₇. By using any adjacent emitter regions E₁ -E₇ as thefirst and second emitters, the device of the present embodiment candetect the optical radiation incident to the device similarly to theprevious embodiments.

The construction of FIG. 32 is advantageous for reducing the parasiticresistance between the adjacent emitters while simultaneously securing asufficient area for photoreception. Further, one can easily form anelectronic circuit such as an amplifier by providing an opaque patternupon selected emitter regions. It should be noted that the structure ofFIG. 32 can be formed easily by a photolithographic patterning process.

Twenty-fourth Embodiment!

FIGS. 33A-33C show various operational states of an opticalsemiconductor device of the open-base multiple emitter HBT constructionaccording to a twenty-fourth embodiment of the present invention.

In the present embodiment, the semiconductor device has a structuresimilar to that of the device of FIG. 26, except that there are providedthree emitter regions, E₁ -E₃, on a common base layer. Similarly asbefore, each of the emitter regions E₁ -E₃ are doped to an impurityconcentration level exceeding the density of state of carriers therein.Thereby, any accumulation of holes in the base layer of the device as aresult of photodetection is successfully eliminated by injectingelectrons from one of the emitters to the base layer by causing aninter-band tunneling.

Referring to FIG. 33A showing a state of the device in which the firstemitter E₁ is grounded and held at a level V_(E1), it will be noted thatthe emitter E₃ is held at a level V_(E3) lower than the ground level.Further, an intermediate voltage V_(E2) is applied to the emitter E₂. Inthe state of FIG. 33A where no optical radiation is applied, thereoccurs no excitation of electrons and holes, and no collector current Icnor a base current flows through the device.

In the state of FIG. 33B, optical radiation is applied to the baseregion B of the device in the state of FIG. 33A. In response to theoptical radiation, there occurs an excitation of electrons and holes inthe base region B, wherein the electrons thus excited are caused to flowto the collector C along the sloped conduction band between the baseregion B and the collector region C in the form of the collector currentIc. Further, the electrons in the base region B are caused to flow tothe emitter region E₃ in the form of a base current.

In response to such a dissipation of electrons, there occurs anaccumulation of the holes in the base region B that acts as a potentialwell against holes. As a result of such an accumulation of the holes inthe base region B, the energy level of the base region B decreases andthe electrons start to flow continuously through the base region B fromthe emitter region E₁ to the collector region C in the form of thecollector current Ic as well as from the emitter region E₁ to theemitter region E₃ in the form of the base current.

FIG. 33C shows the state in which the optical radiation is interruptedin the state of FIG. 33B. In the state of FIG. 33B, the level V_(E2) ofthe second emitter region E₂ is set such that the conduction band of theemitter region E₂ coincides with the valence band of the base region B.Thereby, the electrons are injected from the conduction band of theemitter region E₂ to the valence band of the base region B andannihilate the accumulation of holes therein. Thereby, the deviceresumes the state illustrated in FIG. 33A immediately.

Twenty-fifth Embodiment!

FIGS. 34A-34C show the sampling process conducted by the opticalsemiconductor device having the multiple emitter HBT structure accordingto a twenty-fifth embodiment of the present invention, wherein thesemiconductor device may be the one having the circuit diagram shown inFIG. 24A.

Referring to FIGS. 34A-34C, an input electric signal having a waveformshown in FIG. 34A is sampled by an optical sampling pulse shown in FIG.34B to obtain an output electric signal having the waveform as shown inFIG. 34C, wherein the input electric signal of FIG. 34A has a frequencynf₀, while the optical sampling pulse of FIG. 34B has a frequency ofrepetition of (f₀ -.sub.Δ f), where n is an integer and .sub.Δ f issmaller than f₀ (.sub.Δ f<<f₀). As the sampling frequency is offset fromthe frequency nf₀ of the input electric signal, the output signal ofFIG. 34C has a frequency n.sub.Δ f corresponding to the low speedsampling points given as f₀ /.sub.Δ n. In doing this, the loadresistance is set have a low resistance value such that the change ofthe collector current Ic is reflected in the output voltage.

Similarly to the circuit diagram shown in FIG. 24A, the presentembodiment employs an open-base multiple emitter HBT device having acollector connected to a supply voltage source via a resistance and oneof the emitters connected to the ground. Further, another emitter issupplied with the input electric signal shown in FIG. 34A, and theoptical sampling signal shown in FIG. 34B is supplied to the base regionof the device. The output of the device is obtained at the collector.

Alternatively, one may supply the signal to be sampled to the baseregion B of the device in the form of an optical signal with a signalfrequency nf₀ and further a sampling pulse to the emitter E₂ (FIG. 24A)with a frequency of repetition of f₀ -.sub.Δ f.

Twenty-sixth Embodiment!

FIG. 35 shows the construction of an optical semiconductor deviceaccording to a tenth embodiment of the present invention in a plan view.

Referring to FIG. 35, the semiconductor device includes a number ofopen-base multiple emitter HBT devices arranged in rows and columns,wherein each of the HBT devices includes the first emitter region E₁ andthe second emitter region E₂, in addition to the base region B and thecollector region C similarly to the previous device shown for example inFIG. 26. In the plan view of FIG. 35, the base region B and thecollector region C are not shown.

The devices are formed on a common substrate and electrically isolatedfrom each other by an ion implantation of deep impurities conducted intothe region between the HBT devices as indicated in FIG. 35. Further, theHBT devices belonging to a row are optically isolated from the devicesof other rows, such that each of the input optical beams such as thebeam hν₁ is received commonly by the HBT devices belonging to a row thatcorresponds to the input optical beam.

Further, the first emitter regions E₁ of the HBT devices belonging to acolumn are connected commonly to the ground, while the second emitterregions E₂ of the same column are connected commonly to an electricinput terminal such as E₂₁ corresponding to that column. In other words,there are a number of electric input terminals E₂₁ -E₂₅ incorrespondence to the columns.

In operation, the HBT devices experience a conduction in response to theoptical signal and the electric signal. In other words, it is possibleto select a desired HBT device by the optical signal and the electricsignal. By employing such a selection scheme, it is possible toconstruct a ROM by prohibiting the conduction of selected HBT devicesfor example by conducting an ion implantation process.

In any of the foregoing embodiments described, it is possible to useother semiconductor systems such as GaAs/AlGaAs junction or Si/SiGejunction for the base-emitter junction. Further, it is possible toinduce the desired excitation of electrons and holes by applying anoptical radiation to the collector layer in place of the base layer.Further, the HBT device of the present invention is not limited to thenpn device but one may also use a pnp device.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

What is claimed is:
 1. A light emitting device, comprising:a collectorlayer; a collector electrode connected electrically to said collectorlayer; a base layer provided on said collector layer, said base layerbeing free from a junction region for contacting with an electrode; anemitter layer provided on said base layer, said emitter layer includingat least two, mutually separated emitter regions; and at least twoemitter electrodes provided respectively on said at least two emitterregions.
 2. A light emitting device as claimed in claim 1, wherein eachof said emitter regions has a bandgap substantially exceeding a bandgapof said base layer.
 3. A light emitting device as claimed in claim 1,wherein said base layer has an edge surface covered by ananti-reflection film.
 4. A light emitting device as claimed in claim 1,wherein said light emitting device includes an opaque pattern forshielding said base layer from external optical radiation, between saidfirst and second emitter regions.
 5. A photodetection device,comprising:a collector layer; a collector electrode connectedelectrically to said collector layer; a base layer provided on saidcollector layer, said base layer being free from a junction region forcontacting with an electrode; an emitter layer provided on said baselayer, said emitter layer including at least two, mutually separatedemitter regions; and at least two emitter electrodes providedrespectively on said at least two emitter regions; said base layer beingexposed optically to an external optical radiation.
 6. A photodetectiondevice as claimed in claim 5, wherein said photodetection deviceincludes an optical window for passing said external optical radiationto said base layer.
 7. A photodetection device as claimed in claim 5,wherein said emitter layer forms an optical window between said at leasttwo, mutually separated emitter regions, for passing said externaloptical radiation to said base layer.
 8. A photodetection device asclaimed in claim 5, wherein said base layer carries an anti-reflectionfilm on an edge surface thereof as an optical window for passing saidexternal optical radiation to said base layer.
 9. A photodetectiondevice as claimed in claim 5, wherein said emitter layer carries anopaque pattern on a region thereof locating between said at least twoemitter regions.
 10. A photodetection device as claimed in claim 5,wherein said emitter layer carries an optical filter on a region thereoflocating between said at least two emitter regions.
 11. A photodetectiondevice as claimed in claim 5, wherein at least one of said emitterelectrodes comprises a conductive material transparent to an opticalradiation.
 12. A photodetection device as claimed in claim 5, whereinsaid base layer contains an impurity element with an impurityconcentration level exceeding a density of state of said base layer, andwherein said emitter layer contains an impurity element with an impurityconcentration level exceeding a density of state of said emitter layer.13. A photodetection device as claimed in claim 5, wherein each of saidemitter regions have a bandgap substantially exceeding a bandgap of saidbase layer.
 14. A photodetection device as claimed in claim 5, whereinsaid emitter layer has a bandgap substantially exceeding a bandgap ofsaid base layer, each of said emitter regions having a compositiondifferent from said emitter layer.
 15. A photodetection device asclaimed in claim 5, wherein said at least one of emitter electrodesestablishes a contact with a corresponding emitter region in a statesuch that a substantial part of said emitter region is exposed.
 16. Aphotodetection device as claimed in claim 5, wherein said photodetectionregion further includes a Fabry-Perot resonator.
 17. A method fordetecting an optical beam by a photodetection device, saidphotodetection device including: a collector layer; a collectorelectrode connected electrically to said collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions; and at least two emitter electrodesprovided respectively on said at least two emitter regions; said baselayer being exposed optically to an external optical radiation, saidmethod comprising the steps of:applying a bias voltage across two ofsaid emitter electrodes; and applying an optical beam to said base layeras said external optical radiation, with an optical energy exceeding abandgap of said base layer.
 18. A method for detecting an optical beamby a photodetection device, said photodetection device including: acollector layer; a collector electrode connected electrically to saidcollector layer; a base layer provided on said collector layer, saidbase layer being free from a junction region for contacting with anelectrode; an emitter layer provided on said base layer, said emitterlayer including at least two, mutually separated emitter regions; and atleast two emitter electrodes provided respectively on said at least twoemitter regions; said base layer being exposed optically to an externaloptical radiation, said method comprising the steps of:applying a biasvoltage across one of said emitter electrodes and said collectorelectrode; applying an optical beam to said base layer as said externaloptical radiation, with an optical energy exceeding a bandgap of saidbase layer; and after interrupting said optical beam, injectingelectrons from another emitter electrode to said base layer so as tocancel out accumulation of holes in said base layer.
 19. A semiconductordevice, comprising:a plurality of bipolar transistors arranged in rowsand columns, each of said bipolar transistors comprising: a collectorlayer; a base layer provided on said collector layer, said collectorlayer being free from a junction region for contacting with anelectrode; and an emitter layer provided on said base layer, saidemitter layer including a first emitter region and a second emitterregion; and optical waveguide means for supplying optical signals suchthat, in each row of said bipolar transistors, an optical signal issupplied commonly to said base layers; and said first emitter regions ofsaid plurality of bipolar transistors being connected commonly to theground; in each column of said plurality of bipolar transistors, saidsecond emitter regions being connected commonly to an input terminal; ineach row of said plurality of bipolar transistors, said collector layersbeing connected commonly to a power supply terminal.
 20. A semiconductoroptical integrated circuit, comprising:a light emitting device includinga collector layer, a collector electrode connected electrically to saidcollector layer, a base layer provided on said collector layer, saidbase layer being free from a junction region for contacting with anelectrode, an emitter layer provided on said base layer, said emitterlayer including at least two, mutually separated emitter regions, and atleast two emitter electrodes provided respectively on said at least twoemitter regions, said light emitting device producing an optical beam insaid base layer thereof; and a photodetection device, comprising: acollector layer, a collector electrode connected electrically to saidcollector layer, a base layer provided on said collector layer, saidbase layer being free from a junction region for contacting with anelectrode, an emitter layer provided on said base layer, said emitterlayer including at least two, mutually separated emitter regions, and atleast two emitter electrodes provided respectively on said at least twoemitter regions, said photodetection device detecting an optical beamincident to said base layer thereof; said light emitting device and saidphotodetection device being provided on a common substrate.
 21. Asemiconductor optical integrated circuit as claimed in claim 20, whereinsaid light emitting device and said photodetection device are disposedon said substrate such that said optical beam produced in said baselayer of said light emitting device enters said base layer of saidphotodetection device.
 22. A semiconductor optical switch system,comprising:a semiconductor light emitting device for emitting an opticalbeam; and a semiconductor photodetection device disposed for detectingsaid optical beam; said semiconductor optical switch circuit comprisinga collector layer, a base layer provided on said collector layer, saidbase layer being free from a junction region for contacting with anelectrode, and an emitter layer provided on said base layer, saidemitter layer including at least two, mutually separated emitterregions; said semiconductor photodetection device comprising a collectorlayer, a base layer provided on said collector layer, said base layerbeing free from a junction region for contacting with an electrode, andan emitter layer provided on said base layer, said emitter layerincluding at least two, mutually separated emitter regions.
 23. Asemiconductor optical device comprising:a collector layer; a collectorelectrode connected electrically to said collector layer; a base layerprovided on said collector layer, said base layer being free from ajunction region for contacting with an electrode; an emitter layerprovided on said base layer, said emitter layer including at least two,mutually separated emitter regions; and at least two emitter electrodesprovided respectively on said at least two emitter regions.